There are a variety of applications in which thermoplastic containers are subjected to elevated temperatures. These include hot-fill containers, which must withstand filling with a hot liquid product (for sterilization purposes) without deformation, followed by sealing and a cooling process which produces a vacuum (negative pressure) in the container. Another application is pasteurization--a pasteurizable container is filled and sealed at room temperature, and then exposed to an elevated temperature bath for about ten minutes or longer. The pasteurization process initially imposes high temperatures and positive internal pressures, followed by a cooling process which creates a vacuum in the container. Throughout these procedures, the sealed container must resist deformation so as to remain acceptable in appearance, within a designated volume tolerance, and without leakage. In particular, the threaded neck finish must resist deformation which would prevent a complete seal.
Another high-temperature application is use as a returnable and refillable carbonated beverage container, now commercialized in Europe, South America, and Asia. In this application the container must withstand twenty or more wash and reuse cycles in which it is filled with a carbonated beverage at an elevated pressure, sold to the consumer, returned empty, and washed in a hot caustic solution prior to refilling. These repeated cycles of exposure to hot caustic agents and filling at elevated pressures make it difficult to maintain the threaded neck finish within tolerances required to ensure a good seal.
A number of methods have been proposed for strengthening the neck finish portion of a container to resist deformation at elevated temperatures. One method is to add an additional manufacturing step whereby the neck finish of the preform or container is exposed to a heating element in order to thermally crystallize the neck finish. However, there are several problems with this approach. First, during crystallization the polymer density increases, which produces a volume decrease. Therefore, in order to obtain a desired neck finish dimension, the as-molded dimension must be larger than the final crystallized dimension. It is difficult to achieve close dimensional tolerances with this method. In general, the variability of the critical neck finish dimensions after crystallization are approximately twice that prior to crystallization. Secondly, there is the increased cost of the additional processing step which requires both time and the application of energy (heat). The overall cost of producing a container is very important and tightly controlled because of competitive pressures.
Alternative methods of strengthening the neck finish involve crystallizing select portions of the neck finish, such as the top sealing surface and flange. Again, this requires an additional heating step. Another alternative is to use a high T.sub.g material in one or more layers of the neck finish. Generally, this involves more complex preform injection molding procedures to achieve the necessary layered structure in the finish.
Thus, it would be desirable to provide a thermoplastic preform for a container having a neck finish which resists deformation, particularly at elevated temperatures, and a commercially acceptable method of manufacturing the same.